Silencing means



L. G. WIGLE SILENCING MEANS April 12, 1955 Filed Aug. 30. 195] 2Sheets-Sheet l lM/[NTDR A Fin/lsu' A I rronlvry April l2, 1955 L. G.wlGLE ASILENCING MEANS l 2 Sheets-Sheet 2 Filed Aug. 30. 195] UnitedStates Patent Utilice SILENCING MEANS Lloyd Grant Wigle, Islington,Ontario, Canada, assignor to A. V. Roe Canada Limited, Malton, Gntario,Canada, a corporation Application August 30, 1951, Serial No. 244,425 9Claims. (Cl. 181-43) This invention relates to silencing means andparticularly to means for silencing a discharge of gases from gasturbine engines, rocket motors and the like.

Various constructions of silencing means, such as muiers and soundabsorbent structures, have been used for silencing the discharge ofgases but in the past great difliculty has been experienced inconstructing silencing means which will not affect the indicatedperformance of the device from which the gases are discharged. In thesilencing ot' test cells for gas turbine engines, for example, it is ofgreat importance that the pressure at the inlet of the engine should besubstantially equal to the pressure at the outlet or discharge nozzle;otherwise misleading readings of the engine thrust will be obtained onthe test equipment.

Furthermore the gases discharged from reaction propulsion motors areusually at a very high temperature and conventional silencing means arelikely to deteriorate rapidly under such conditions.

The main object of this invention is the provision of a silencing meanswhich will have substantially no effect upon the indicated performanceof the gas turbine engine or other reaction propulsion device to whichit is applied.

Another object of the invention is the cooling of the discharged gasesso that they will not destroy the means provided for silencing them.Other objects and advantages will become apparent during the course otthe following description of a preferred application of the invention.

In the attached drawings forming a part of this application and in whichlike characters of reference are used to denote like parts throughoutthe same:

Fig. l is a sectional side elevation of a gas turbine test cell andsilencing device according to this invention, showing an engine undertest therein;

Fig. 2 is a sectional plan View of the test cell and silencing devicetaken on the line 2-2 in Fig. 1;

Fig. 3 is a sectional end elevation of the test cell and silencingdevice taken on the line 3 3 in Fig. 1; and

Fig. 4 is a perspective view of the sound absorbing vanes in thesilencing house.

In the drawings the test cell is shown cnt away at its inlet end sincethe silencing of the inlet is the subject of my copending applicationNo. 224,080, dated May 2, 1951, now abandoned, and has no bearing uponthis invention. The engine 11 is shown mounted on the usual balances 12in the test cell and its discharge nozzle 11a protrudes into thesilencer which comprises principally the silencing house 13 and theconduit 14. The house 13 is insulated from the adjacent wall of the testcell by the air gap 15 and the sponge rubber sealing ring 16. Thedischarge nozzle 11a is surrounded by a bulkhead 17 in the wall of thetest cell and it will be understood that, to obtain accurate readings ofthe performance of the engine, the pressures on both sides of this bathemust be substantially equal.

A mixing tube 18 is mounted in the house 13 so that the distance betweenthe plane of its inlet and the said nozzle is about 11/2 times thediameter of the discharge nozzle and its outlet is arranged to dischargeinto the larger conduit 14. The mixing tube 1S is a tube of heavy steelplate or similar material while the inner walls 14a of the conduit 14,in accordance with known sound control practice, are made of relativelylight gauge sheet metal lined with a sound absorbing medium such as wirewool with an inner lining of a suitable perforated sheet to hold thesound absorbing medium in place; the walls of 2,706,013 Patented Apr.12, 1955 the conduit 14 are therefore necessarily subject to damage fromexcessive heat.

The inlet of the mixing tube is specially constructed and embodies alianged frusto-conical inlet-defining member 19 supported by spacers 20in spaced relationship to a flange 18a on the periphery of the tube, sothat an air passage 21 exists between the orice member 19 and theforward edge of the tube. The smaller diameter of the conical frustum,which forms the orifice, is arranged so that it is substantially equalto the diameter in the plane of the orifice of the diver-ging cone ofdischarged exhaust gases from the engine; the orifice therefore conformsto the perimeter of the stream of gases which it is to accommodate.

Peripherally arranged around the outlet end of the mixing tube 18 is aflange 22, restricting the annular space between the outlet terminalportion of the mixing tube and the overlapping inlet terminal portion ofthe conduit 14, and leaving an air gap 23.

As will be seen from Fig. l, the conduit 14 protrudes inwardly throughthe wall of the house 13 and a narrow annular air gap 24 is providedbetween the inlet terminal portion of the conduit and the surroundingwall of the house. The dimensions of this air gap are chosen so that itsaxial length is many times greater than its radial thickness.

The principal inlet of air to the house 13 is effected through an airgap 25 under the eaves of the roof of the house. To control and silencethe flow of air from this gap through the inlet passage provided by thehouse 13, three sets of inclined rectangular vanes 26 of sound absorbingconstruction, in accordance with known practice, are situated in seriesin the upper part of the structure, the vanes in each set extending thefull width of the house and being hinged to its walls at their loweredges. The upper edges of the vanes in each set are interconnected byconnecting links 27 each embodying a jack screw 27' bearing against thewall of the house 13 and whereby the incidence of the vanes may be setto control the pressure in the lower part of the house 13, as will beexplained hereinunder, by reducing or increasing the rate of flow of airthrough the air inlet passage provided by the house 13.

A duct 28 penetrating the bulkhead 17 above the discharge nozzle 11a ofthe engine is led across the house 13 and into the conduit 14 throughthe annular gap 23 between the inner surface of the conduit and theouter peripheral rim of the flange 22. In some installations thisannular gap 23 may be so narrow as to necessitate two or more ducts 28annularly arranged around the upper part of the bulkhead 17 in order toprovide sufficient capacity; however, in certain cases, orifices in thebulkhead 17 will provide suiiicient ventilation to allow the duct orducts to be dispensed with.

In operation the engine 11 under test discharges a conical jet ofexhaust gases which substantially fills the orifice of the member 19.These gases are travelling across the discontinuity between thedischarge nozzle and the mixing tube at a very high velocity and are athigh temperature. Since the jet substantially fills the orifice, no aircan be entrained and carried into the mixing tube through the orifice sothat the ambient air between the nozzle and the mixing tube is notseriously disturbed. It has been found that if such conditions existover a distance approximately 11/2 times the diameter of the nozzle theeffective thrust of the jet is substantially unmodified. Cooling air isintroduced into the mixing tube annularly through the air passage 21immediately downstream of the orice and this cooling air is mixed withthe exhaust gases in the mixing tube 18. The cooling air tends to adhereto some extent to the walls of the mixing tube thereby protecting themixing tube from overheating, though in any case the tube is made ofsuch heavy plate that it can withstand considerable heat without illeffects. The cooling of the exhaust gases causes them to contract sothat both their speed and temperature are materially reduced as they aredischarged from the mixing tube into the conduit 14. Additional air isdrawn into this conduit, at the discontinuity between the mixing tubeand the conduit, through the annular air gap 23, and this additional airfurther cools the exhaust gases and again tends to adhere as a coolingfilm to the walls of the conduit to protect them against overheating. Inconsequence the exhaust gases, after the two stage introduction ofcooling air, are greatly decelerated before their eventual discharge toatmosphere from the conduit 14.

It will be recognized by those skilled in the art that the cooling anddeceleration of the exhaust gases by the introduction of air may beaugmented by the introduction of water sprayed into the gas stream, butin general, with the proper control of the admission of cooling air, theconventional water sprays, with their piping and other undesirablecomplications, may be dispensed with. According to this invention thesound absorbing walls of the conduit 14 are protected againstoverheating by the pre-cooling of the hot gases in the mixing tube 18and by the lrn cooling effect of the additional air introduced throughthe annular gap 23, and this combination is adequate without therecourse to liquid cooling media.

Air for cooling is admitted into the house 13 through the air gaps 25and the sets of vanes 26, which are arranged obliquely to bathe andrestrict sound waves and their sound absorbing construction assists insilencing any noise arising from the discharge of gases from the engineand the introduction of cooling air through the air passages 21 and 23.It is of great importance that the pressure in the lower part of thesilencing house 13 should be substantially the same as the pressure inthe test cell in order to avoid interference with the indications ofperformance of the propulsive jet. The vanes 26 are thereforeadjustablev about the hinges at their lower edges and by variation oftheir incidence, through the medium of the adjustable links 27, thepressure in the lower part of the house 13 can be increased ordecreased. In actual practice it is desirable that the pressure in theneighbourhood of the discharge nozzle 11a in the house 13 should beslightly lower (by one or two inches of water) than the pressure in thetest house in order to insure that no gases from the silencer house 13tend to enter the test house 10.

The performance of the engine may also be affected by the circulation ofhot gases in the test cell; these gases may be heated simply by contactwith the hot parts of the engine under test. The duct 28 is thereforeprovided to ventilate the test cell and to carry away any hot gaseswhich may arise from around the engine. A depression of the order of 100inches of water exists between the pressure in the test cell and theextractor pressure at the outlet end of the duct 2S, in the conduit 14,and the diameter of the duct must therefore be chosen, in accordancewith the dimensions of the test cell, to insure adequate ventilationthereof. In a typical example the quantity of air extracted through theduct was found to be of the order of 1/3 of the mass flow of the engine.If a single circular duct cannot be accommodated, a plurality of ductsmay be annularly arranged around the upper part of the bulkhead 17 asmentioned previously in this specification.

It will be noted that care is taken to insulate the Various componentsof the silencer from one another; the house 13 is insulated from thetest cell by the air gap and the conduit 14 is insulated from the house13 by the air gap 24. Although the gap 15 is reinforced by the soundabsorbing sealing ring 16, both the air gaps 15 and 24 are relativelynarrow. so that any sound waves tending to escape through them areretiected between the adjacent surfaces and considerably absorbed beforethey can escape.

It will be understood that the width of the air gap 21 controlled by thespacers 20, the width of the air gap 23 controlled by the iiange 22, theincidence of the vanes 26 adjustable by the links 27, and the capacityof the duct 28 must be determined empirically to produce the bestresults for the particular type of engine under test. It has been foundthat for an engine discharge of W pounds per second, satisfactoryresults can be obtained by the introduction of cooling air at W/2 poundsper second through each of the gaps 21 and 23 and through the duct 28,so that a flow of 3W/ 2 pounds per second passes through the mixing tubeand 5W/2 passes through the conduit. However experiment has indicatedthat none cf these quantities is particularly critical. It will beunderstood therefore that the form of the invention herewith shown anddescribed is to be taken as a preferred example of the same and thatvarious changes in the shape, size and arrangement of the parts may beresorted to without departing from the spirit of the invention or thescope of the claims.

What I claim as my invention is:

1. A silencer comprising, a nozzle through which a stream of hot gasesis discharged at high velocity, an open-ended tube having one endaxially spaced from the outlet of the nozzle and into which the streamof gases is discharged, an open-ended conduit discontinuous with thetube and having one end adjacent the other end of the tube to receivegases discharged from the tube, the conduit having a sound absorbinglining, a housing enclosing and extending between the outlet of thenozzle and the said end of the conduit, an inlet opening in the housingremote from the nozzle, the housing providing an air inlet passageleading inward from the opening, and sound absorbing vanes obliquelydisposed in the said passage.

2. A silencer comprising, a nozzle through which a stream of hot gasesis discharged at high velocity, an open-ended tube having one end facingand axially spaced from the outlet of the nozzle and into which thestream of gases is discharged, an open-ended conduit into which theother end of the tube extends in spaced relationship therewith, a soundabsorbing lining in the conduit, a housing enclosing the tube and havingopenings receiving the nozzle and the conduit, the edges of the openingsbeing spaced from the nozzle and the conduit to provide sound insulatingair gaps between the said edges and the nozzle and conduit, an inletopening in the housing, the housing providing an air inlet passageleading inwardly from the opening, and sound absorbing surfaces disposedin the said passage.

3. A test cell installation for a reaction propulsion motor comprising atest cell having a port for the cjection nozzle of the motor, asilencing house adjacent the port and spaced from the test cell andhaving an opening facing the port in the test cell, an open-ended tubein the silencing house and having one end axially spaced from and facingthe port in the test cell to receive gases discharged from the nozzle,an open-ended conduit discontinuous with and extending from the otherend of the tube through one wall of the silencing house to receive gasesdischarged from the tube, the conduit having sound absorbing inner wallsand having its outer periphery spaced from the said wall of thesilencing house, an inlet opening in the silencing house, the silencinghouse providing an air inlet passage leading inwardly from the inletopening, and sound absorbing means disposed in the said passage.

4. A test cell installation as claimed in claim 3 including meansplacing the interior of the test cell in communication with the interiorof the conduit to ventilate the test cell into the conduit.

5. A test cell installation as claimed in claim 3 includi ing a ductconnecting the interior of the test cell with the interior of theconduit to ventilate the test cell into the conduit.

6. A silencer comprising, a nozzle through which a stream of hot gasesis discharged at high velocity, an open-ended tube having an inlet endfacing the outlet of the nozzle and into which the stream of gases isdischarged, an orifice-defining member axially spaced from the outlet ofthe nozzle and from the inlet end of the tube and having an orifice forthe stream of gases substantially conforming to the cross-sectionalperimeter of the said stream in the plane of the orifice, the spacebetween the orifice-defining member and the inlet end of the tubeproviding an inlet for air to cool the stream of gases in the tube, anopen-ended conduit disposed co-axially with respect to the said otherend of the tube, one end of the conduit surrounding and being annularlyspaced from said other end of the tube to provide an inlet for airbetween the tube and the conduit to cool the stream of gases in theconduit, a housing enclosing the tube and having openings in oppositelyfacing walls through which pass the nozzle and the conduit, an inletopening in the housing, the housing providing an air inlet passageleading inwardly from the opening, and sound absorbing surfaces disposedin the said passage.

7. A silencer comprising, a nozzle of a reaction propulsion motorthrough which a stream of hot gases is discharged at high velocity, anopen-ended tube having an inlet end facing the outlet of the nozzle andinto which the stream of gases is discharged, an orifice-defining memberaxially spaced from the outlet of the nozzle and from ill@ inlt end ofthe tube and having an orifice for the stream of gases Isubstantiallyconforming to the crosssectional perimeter of the said stream in theplane of the orifice, the space-between the orice-defining member andthe inlet end of the tube providing an inlet for air to cool the streamof gases in the tube, an open-ended conduit disposed co-axially withrespect to the said other end of the tube, one end of the conduitsurrounding and being annularly spaced from said other end of the tubeto provide an inlet for air between the tube and the conduit to cool thestream of gases in the conduit, an annular flange surrounding the saidother end of the tube, the flange being mounted on one and spaced fromthe other of the said other end of the tube and the inner periphery ofthe said one end of the conduit to restrict the size of the said inletfor air, a housing enclosing the tube and having openings in oppositelyfacing walls through which pass the nozzle and the conduit, an inletopening in the housing, the housing providing an air inlet passageleading inwardly from the inlet opening, and sound absorbing surfacesdisposed in the said passage.

8. A test cell installation for a reaction propulsion motor comprising atest cell enclosing the motor and having a port for the ejection nozzleof the motor, a silencing house adjacent the port and having an openingproviding communication with the test cell, and an openended tube in thesilencing house and having one end axially spaced from and facing theport in the test cell to receive gases discharged from the nozzlethrough the opening in the silencing house, and an open-ended conduitdiscontinuous with and extending from the other end of the tube throughone wall of the silencing house to receive gases discharged from thetube, an inlet opening in the silencing house walls defining an airinlet passage leading inwardly from the inlet opening, and soundabsorbing vanes disposed in the said passage for silencing it, the vanesbeing disposed obliquely with respect to the flow of air through thesaid passage and being pivoted to the said walls for adjustment of theangle of incidence of the vanes with respect to the said ow of air.

9. In a silencer, a nozzle through which a stream of gases isdischarged, an open-ended tube having an inlet end axially spaced fromthe outlet of the nozzle and into which the stream of gases isdischarged, the cross-sectional area of the said inlet end beingmaterially greater than the cross-sectional area of the stream of gasesin the plane of the said inlet end, an orice-dening member at the inletend of the tube having a generally radially inwardly extending flangedefining an orifice substantially conforming to the cross-sectionalperimeter of the stream of gases in the plane of the orifice, and a sideinlet leading into the tube downstream of the orifice definingmember topermit the entry of air into the tube to cool the stream of gases in thetube.

References Cited in the file of this patent UNITED STATES PATENTS1,658,402 Warth Feb. 7, 1928 1,794,276 Bowes Feb. 24, 1931 2,150,530Warsing Mar. 14, 1939 2,270,825 Parkinson et al. Jan. 20, 1942 2,450,212Thomas Sept. 28, 1948 FOREIGN PATENTS 119,301 Switzerland Mar. 1, 1927249,518 Great Britain June 9, 1927 720,368 France Feb. 18, 1932 536,848Germany Oct. 28, 1931 312,994 Italy Dec. 1, 1933 314,290 Italy Jan. 22,1934 817,517 France Sept. 4, 1937

